OTTAWA — Canada Post suspended delivery-time guarantees to its customers Tuesday, acknowledging a lack of progress in contract talks with its unionized workforce Tuesday as a fourth week of rotating strikes began.

The Crown corporation also warned its customers that more delivery delays are inevitable after the Canadian Union of Postal Workers closed two major processing centres in Ontario and British Columbia.

"Despite lengthy discussions and continued proposals by Canada Post to respond to the union’s demands through three rounds of mediation, we are still no closer to a deal," the agency said in an emailed statement. "Unfortunately, the growing impact of the union’s rotating strikes on postal operations across the country means Canada Post must suspend its service-delivery guarantees until further notice."

CUPW members walked off the job early Tuesday in Toronto and at a distribution centre in Scarborough, Ont.

Rotating strikes also resumed late Monday at locations in Nova Scotia, including Halifax, but ended Tuesday morning at a processing centre near Vancouver.

The Vancouver and Toronto shutdowns will cause significant service disruptions and add to a backlog of trailers filled with parcels that have yet to be sorted for distribution, Canada Post said.

"We will continue to make best efforts to deliver, but the union's strike efforts have caused significant backlogs and delays throughout our network."

More than 180 trailers containing parcels and mail were idled in Toronto alone as of Tuesday, the post office said.

CUPW national president Mike Palecek said that contract talks made little progress in recent weeks despite the involvement of a special mediator whose mandate expired on the weekend.

"In spite of the continued assistance of the mediator over the weekend, Canada Post still refuses to address our major issues of health and safety, staffing, over-burdening, job security, a reduction in precarious employment, fair wages for all and a better work-life balance," Palecek said in a statement. "While we remain at the bargaining table, ready to negotiate with Canada Post, we will not sit back in silence. This fight is not over."

Canada Post rejected the union's claim, repeating that it has made "significant" wage, benefit and job-security proposals.

Bargaining for contracts for two groups of employees — urban carriers and rural and suburban workers — has been ongoing for nearly a year.

Last week, Prime Minister Justin Trudeau warned his government could intervene if progress isn't made soon in the talks, but did not say what action might be taken.

A spokesperson for Labour Minister Patty Hajdu repeated the government's warning Monday that it might step in ahead of the holiday online shopping rush, but again provided no details.

There have been rotating postal strikes across Canada since Oct. 22 involving most of the union's 50,000 members.

The last time the federal government forced an end to a work dispute at Canada Post was in 2011, when the former Conservative government passed back-to-work legislation to end a two-week lockout by the Crown agency.